Drilling fluids

ABSTRACT

This invention relates to drilling fluids, to drilling lubricants for such drilling fluids and to methods of drilling wells, particularly oil and/or gas wells using such drilling fluids. In particular, the invention relates to the inclusion of lubricants which afford lubrication in drilling fluids in which the lubricant compatibility and/or performance may be adversely affected by other components of the drilling fluid, particularly divalent cations such as calcium, formate salts or highly alkaline components such as alkali silicate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/851,657, filed May 24, 2004, which has been allowed. Thisapplication, in its entirety, is incorporated herein by reference.

SUMMARY OF THE INVENTION

This invention relates to drilling fluids, to drilling lubricants forsuch drilling fluids and to methods of drilling wells, particularly oiland/or gas wells using such drilling fluids. In particular the inventionrelates to the inclusion of lubricants which afford lubrication indrilling fluids in which lubricant compatibility and/or performance maybe adversely affected by other components of the drilling fluid,particularly divalent cations such as calcium, formate salts or highlyalkaline components such as alkali silicate.

BACKGROUND

Drilling holes in the ground is often undertaken in searching for oil orother hydrocarbon sources. In such drilling it is usual to provide adrilling fluid (sometimes called a drilling mud) circulated down thehole being drilled e.g. within the drill string, around the drill bitand returning to the surface. The drilling fluid serves to cool andlubricate the bit and to carry drilling cuttings away from the bit. Thehydrostatic pressure exerted by the drilling fluid is often also used tobalance pressure within the rock formation through which the hole isbeing drilled, most notably when a pressurised fluid e.g. water, oil orgas, is contained within the rock. Commonly water, often as a brine, isthe main fluid component of the drilling fluid. Particularly withaqueous drilling fluids, lubricant additives are commonly included inthe fluid to improve the lubrication properties of the fluid,particularly to improve lubrication of or around the drill bit.

We have found that the inclusion of certain hydrocarbyl polyetherphosphate esters in combination with polyalkylene glycols, can giveaqueous drilling fluids which provide good lubricating properties in awide range of drilling fluids, in particular where the drilling fluidhas a brine phase which is sufficiently concentrated and/or includesmaterials which can cause incompatibility with conventional lubricants.Incompatibility can be relevant in commercially important drillingfluids, and can arise with concentrated brines, particularly saturatedor near saturated brines or with the presence of materials such ascalcium, particularly as calcium chloride; formate salts; and solublesilicate salts which can be used in very highly alkaline brines.

DETAILED DESCRIPTION

The invention accordingly provides a drilling fluid having an aqueousbrine phase which includes at least one of:

-   a) dissolved calcium, usually as Ca²⁺ ions, particularly at a    concentration of at least 0.1 molar,-   b) dissolved formate, usually as a potassium or, and particularly,    caesium salt, (caesium formate), and particularly at a concentration    of at least 0.5 molar;-   c) dissolved silicate, usually provided as alkali metal, especially    sodium, silicate, particularly at a concentration of at least 0.4    molar; or-   d) brine salts at a concentration of at least 90% by weight of the    concentration at saturation;    and further including at least one polyalkylene glycol and a    lubricating agent which comprises at least one hydrocarbyl polyether    phosphate.

The invention includes a method of drilling a well bore in which adrilling fluid is circulated down a drill string extending through awell bore, around a drill bit and returns in the space between the drillstring and the wall of the well bore, in which the drilling fluid has abrine aqueous phase which includes at least one of:

-   a) dissolved calcium, usually as Ca²⁺ ions, particularly at a    concentration of at least 0.1 molar,-   b) dissolved formate, usually as a potassium or, and particularly,    caesium salt, (caesium formate), and particularly at a concentration    of at least 0.5 molar;-   c) dissolved silicate, usually provided as alkali metal, especially    sodium, silicate, particularly at a concentration of at least 0.4    molar; or-   d) brine salt(s) at a concentration of at least 90% by weight of the    concentration at saturation;    and further including at least one polyalkylene glycol and a    lubricating agent which comprises at least one hydrocarbyl polyether    phosphate.

The invention particularly provides drilling fluids including such brineformulation components and accordingly the invention includes thefollowing aspects:

-   i) A drilling fluid based on an aqueous brine containing calcium at    a concentration of at least 0.1 molar and which includes at least    one polyalkylene glycol and a lubricating agent which comprises at    least one hydrocarbyl polyether phosphate.-   ii) A drilling fluid based on an aqueous brine containing formate,    particularly as potassium or caesium formate, at a concentration of    at least 0.5 molar and which includes at least one polyalkylene    glycol and a lubricating agent which comprises at least one    hydrocarbyl polyether phosphate.-   iii) A drilling fluid based on an aqueous brine containing soluble    silicate at a concentration of at least 0.4 molar and which includes    at least one polyalkylene glycol and as a lubricating agent which    comprises at least one hydrocarbyl polyether phosphate.-   iv) A drilling fluid based on an aqueous brine containing brine    salt(s) at a concentration of at least 90% by weight of the    concentration at saturation and which includes at least one    polyalkylene glycol and as a lubricating agent which comprises at    least one hydrocarbyl polyether phosphate.

The invention further particularly provides methods of drilling usingdrilling fluids including such brine formulation components andaccordingly the invention includes the following aspects:

-   i) A method of drilling which uses a drilling fluid based on a brine    containing calcium at a concentration of at least 0.1 molar and    which includes at least one polyalkylene glycol and as a lubricating    agent at least one hydrocarbyl polyether phosphate.-   ii) A method of drilling which uses a drilling fluid based on a    brine formate, particularly as potassium or caesium formate, at a    concentration of at least 0.5 molar and which includes at least one    polyalkylene glycol and as a lubricating agent at least one    hydrocarbyl polyether phosphate.-   iii) A method of drilling which uses a drilling fluid based on a    brine containing soluble silicate at a concentration of at least 0.4    molar and which includes at least one polyalkylene glycol and as a    lubricating agent at least one hydrocarbyl polyether phosphate.-   iv) A method of drilling, particularly a drill-in method, which uses    a drilling fluid based on a brine containing brine salt(s) at a    concentration of at least 90% by weight of the concentration at    saturation and which includes at least one polyalkylene glycol and    as a lubricating agent which comprises at least one hydrocarbyl    polyether phosphate

The hydrocarbyl polyether phosphates used as lubricants in the presentinvention can be formulated as lubricant formulations for inclusion indrilling fluids as solutions in aqueous mixtures including polyalkyleneglycols. Particularly valuable aqueous lubricant formulations for use inthis invention are those based on alkyl polyether mono-phosphate estersand polyethylene glycols and the invention accordingly includes:

-   i) A lubricating agent for a drilling fluid which comprises an    aqueous solution of at least one mono-(hydrocarbyl polyether)    phosphate ester, particularly at a concentration of from 2 to 40 wt    % of the solution, and at least one polyethylene glycol having an    average molecular weight of from 100 to 4000, particularly at a    concentration of from 50 to 90 wt % of the solution;-   ii) A method of drilling a well bore in which a drilling fluid is    circulated through a drill string extending through a well bore,    around a drill bit and returns in the space between the drill string    and the wall of the well bore, in which the drilling fluid has an    aqueous phase which includes at least one mono-(hydrocarbyl    polyether) phosphate ester, particularly at a concentration of from    2 to 40 wt % of the drilling fluid, and at least one polyethylene    glycol having an average molecular weight of from 100 to 4000,    particularly at a concentration of from 50 to 90 wt % of the    drilling fluid.

The amount of lubricant formulation that is included in the drillingfluid is typically from 0.5 to 10%, more usually from 1 to 5 wt % of thedrilling fluid. This gives corresponding typical levels of the phosphateester and PAG of approximately 0.05 to 1 and 0.3 to 8 more usually from0.1 to 0.5 and 0.7 to 4 all % by weight of the drilling fluid.

In this invention the term “brine” refers to an aqueous solution of oneor more salts, particularly one in which the salt concentration isrelatively high, in particular to provide a solution having a densitysubstantially higher than water. Commonly higher concentrations of saltsthan present in normal seawater are used to achieve higher densities. Inthis context a saturated brine is one which, under use conditions, willnot dissolve further of at least one (but, where more than one salt isused, not necessarily all) of its component salt(s) and references topercent saturation are to the relative concentration of such a salt ascompared with its concentration at saturation.

The term “drilling fluid” refer to water based fluids which may (andusually will) contain dispersed solids and which are used in drillingwell bores to provide cooling, lubrication, pressure balance and/ordrilling cuttings removal (commonly all of these) and may additionallyact as a carrier for additives having particular effects e.g. shaleinhibition. In common use in the art the terms “drilling fluid” and“drilling mud” are synonymous. Very commonly drilling fluids includedispersed solids, usually either weighting solids [relatively highdensity finely divided solid material used to increase the density of adrilling fluid particularly to increase the hydrostatic pressure at thebottom of a well bore] or solids having other functions such as bridgingmaterials [finely divided materials used to prevent or eliminate loss offluid from the well bore to the formation, (lost circulation) or solidsincluded in a drilling fluid to bridge across the pore throats orfractures of an exposed rock thereby building a filter cake to preventloss of whole mud or excessive filtrate]. Thus, drilling fluids used indrilling the main, typically mainly vertical, bore usually includedispersed solid weighting materials and are referred to as “weighteddrilling fluids”.

Weighting materials are typically relatively dense solids that areinsoluble in the drilling fluid continuous phase and are used toincrease the density of a drilling fluid. Examples include e.g. barites,BaSO₄ with a specific gravity (SG) typically 4.2 or higher; and ironoxide, particularly haematite, SG typically at least 5; are commonlyused, but galena (PbS), SG typically about 7.5; calcium carbonate, SGtypically 2.7 to 2.8; siderite, ferrous carbonate—SG typically about3.8; and ilmenite, mixed iron oxide titanium dioxide, SG typically about4.6; have also been used. In this context SG is numericallysubstantially the same as density measure in g.cm⁻³. When used theamount of weighting agent used in a drilling fluid is typically in therange 1 to 75%, more usually 2 to 50%, particularly 5 to 30%, moreparticularly 10 to 20%, by weight of drilling fluid.

The invention accordingly provides a weighted drilling fluid having anaqueous brine phase which includes at least one of:

-   a) dissolved calcium, usually as Ca²⁺ ions, particularly at a    concentration of at least 0.1 molar,-   b) dissolved formate, usually as a potassium or, and particularly,    caesium salt, (caesium formate), and particularly at a concentration    of at least 0.5 molar;-   c) dissolved silicate, usually provided as alkali metal, especially    sodium, silicate, particularly at a concentration of at least 0.4    molar; or-   d) brine salts at a concentration of at least 90% by weight of the    concentration at saturation;    and further including at least one polyalkylene glycol and a    lubricating agent which comprises at least one hydrocarbyl polyether    phosphate, and finely divided solid bridging material.

The invention includes a method of drilling a well bore down, usuallysubstantially vertically, to a target, particularly a production,formation in which a weighted drilling fluid is circulated down a drillstring extending down a well bore, around a drill bit and upwardly inthe space between the drill string and the wall of the well bore, inwhich the drilling fluid has a brine aqueous phase which includes atleast one of:

-   a) dissolved calcium, usually as Ca²⁺ ions, particularly at a    concentration of at least 0.1 molar,-   b) dissolved formate, usually as a potassium or, and particularly,    caesium salt, (caesium formate), and particularly at a concentration    of at least 0.5 molar;-   c) dissolved silicate, usually provided as alkali metal, especially    sodium, silicate, particularly at a concentration of at least 0.4    molar; or-   d) brine salts at a concentration of at least 90% by weight of the    concentration at saturation;    and further including at least one polyalkylene glycol and a    lubricating agent which comprises at least one hydrocarbyl polyether    phosphate, and finely divided solid weighting material.

A somewhat different type of drilling fluid is a “drill-in” fluid, whichis a fluid used when drilling through the reservoir section of a wellbore e.g. from the bottom of a primary well in a reservoir to provideaccess to more remote parts of the reservoir. Frequently such well boresare typically mainly horizontal. Drill-in fluids usually includedispersed bridging materials i.e. solids that can block the pores in thereservoir rock to prevent flow of oil or gas into the well bore alongits length, particularly during drilling. Commonly it is desirable to beable to remove the bridging material after drilling e.g. to enableproduction from along the a length of the well bore, and with this inmind bridging materials are commonly water soluble salts e.g. sodiumchloride, used in drilling with saturated brine based drilling fluids,or calcium carbonate. Such bridging materials can subsequently beremoved using water or acid respectively. Typically the amount ofbridging agent in a drill-in fluid is from 0.1 to 30%, particularly 1 to5%, by weight of the drilling fluid.

The invention accordingly provides a drill-in fluid having an aqueousbrine phase which includes at least one of:

-   a) dissolved calcium, usually as Ca²⁺ ions, particularly at a    concentration of at least 0.1 molar,-   b) dissolved formate, usually as a potassium or, and particularly,    caesium salt, (caesium formate), and particularly at a concentration    of at least 0.5 molar;-   c) dissolved silicate, usually provided as alkali metal, especially    sodium, silicate, particularly at a concentration of at least 0.4    molar; or-   d) brine salt(s) at a concentration of at least 90% by weight of the    concentration at saturation;    and further including at least one polyalkylene glycol and a    lubricating agent which comprises at least one hydrocarbyl polyether    phosphate, and finely divided solid weighting material.

The invention includes a method of drilling-in a well bore in aproduction formation, particularly substantially horizontally in which adrilling fluid in the form of a drill-in fluid is circulated through adrill string extending along the well bore, around a drill bit andreturns in the space between the drill string and the wall of the wellbore, in which the drill-in fluid has a brine aqueous phase whichincludes at least one of:

-   a) dissolved calcium, usually as Ca²⁺ ions, particularly at a    concentration of at least 0.1 molar,-   b) dissolved formate, usually as a potassium or, and particularly,    caesium salt, (caesium formate), and particularly at a concentration    of at least 0.5 molar;-   c) dissolved silicate, usually provided as alkali metal, especially    sodium, silicate, particularly at a concentration of at least 0.4    molar; or-   d) brine salt(s) at a concentration of at least 90% by weight of the    concentration at saturation;    and further including at least one polyalkylene glycol and a    lubricating agent which comprises at least one hydrocarbyl polyether    phosphate, and finely divided solid bridging material.

For drilling-in fluids and methods, type a) and d) brines are morecommonly used than the b) and c) type brines.

The hydrocarbyl polyether phosphates used as lubricants in the productsand methods of this invention are desirably of the formula (I):

R¹O.(AO)_(n).P(O)(OH)₂  (I)

whereR¹ is a C₆ to C₃₀ straight or branched chain alkyl, alkenyl oralkadienyl group;AO is an alkyleneoxy group;n is from 2 to 100.

Within the ranges for the substituents of the compounds of the formula(I) above R¹, is desirably a C₈ to C₂₂, particularly a C₈ to C₂₀, moreparticularly C₈ to C₁₈, especially a C₁₂ to C₁₈ straight or branchedchain alkyl, alkenyl or alkadienyl group.

Desirably all the alkyleneoxy groups, AO, are ethyleneoxy (—C₂H₄O—)groups, though mixtures of ethyleneoxy and propyleneoxy (—C₃H₆O—)groups, desirably having a molar ratio of ethyleneoxy groups topropyleneoxy groups of from 3:1 to 20:1 (lower proportions ofpropyleneoxy groups give PAGs which are not readily distinguishable fromPEG homopolymers), more usually at least 5:1 and desirably at least10:1, may be used. When the alkyleneoxy groups are mixed ethyleneoxy andpropyleneoxy groups, the polyalkyleneoxy chain can be a random or blockcopolymeric chain. Within the range 2 to 100, n is desirably at least 3,more desirably from 4 to 50, particularly 5 to 20. Generally the valueof n will be chosen to be larger where the number of carbon atoms in thegroup R¹ is larger within the ranges set out above. The number of unitsin the polyoxyalkylene chain, ‘n’, is itself an average value and may benon-integral.

In use in drilling well bores, the drilling fluid may well be alkali,often having a pH above 9, commonly above 10 and sometimes above 11.Under such conditions, it is likely that the phosphate mono-esters usedin the invention will be present at least in part as salts of basespecies also present in the drilling fluid. Such salt forming speciescan include alkali metals such as sodium, potassium and caesium, andalkali earth metals such as calcium.

The hydrocarbyl polyether phosphate esters used in the invention aredesirable mono-hydrocarbyl polyether esters. Generally such esters areavailable as mixtures of mono- and di-esters and in the invention theproportion of mono-ester will usually be at least 50 mole %, desirablyat least 60 mole %, and may be above 70 mole % of the phosphate ester.Typical synthetic routes to such esters, involve reacting a precursorhydrocarbyl polyether (usually an alkyl, alkenyl or alkadienylpolyalkoxylate, usually a polyethoxylate) with a phosphating agent suchas phosphorus pentoxide or polyphosphoric acid. The use ofpolyphosphoric acid in the synthesis can give higher proportions ofmono-ester than are typically obtained with phosphorus pentoxide andproducts made using polyphosphoric acid may, thus, be preferred for usein this invention.

The polyalkylene glycol (PAG) is desirably a polyethylene glycol (PEG)as PEGs generally give better compatibility of the phosphate ester inthe lubricant formulation and give lubricants with greater fluidity(lower viscosity). PAGs having minor proportions of other alkyleneglycol residues, particularly propylene glycol residues, may be used butin this case the proportion of other alkylene glycol residues,particularly propylene glycol residues is desirably not more than about10 mole %, particularly not more than about 5 mole %. The PAGs,particularly PEGs, used in the invention typically have a number averagemolecular weight of at least 100 and desirably not more than about 4000,with optimum performance in terms of providing compatibility in thedrilling fluid, especially in brines, in the range of 200 to 1000 e.g.about 400.

The proportions of the major components of the lubricant formulations ofand used in this invention are typically as follows:

Amount (% w/w) Material Broad More usually polyalkylene glycol,particularly PEG 50 to 90  60 to 80 hydrocarbyl polyether phosphate 2 to40  5 to 20 ester Water to 100% but usually: 5 to 50 10 to 30

As is noted above, the aqueous phase of the drilling fluids will usuallybe brines in which the salts used to form the brine are typicallyelectrolytes. Commonly lubricants are not compatible with some or allbrines containing relatively high concentrations of electrolyte, inparticular brines containing one or more of:

-   a) dissolved calcium, usually as Ca²⁺ ions, particularly at a    concentration of at least 0.1 molar,-   b) dissolved formate, usually as a potassium or, and particularly,    caesium salt, (caesium formate), and particularly at a concentration    of at least 0.5 molar;-   c) dissolved silicate, usually provided as alkali metal, especially    sodium, silicate, particularly at a concentration of at least 0.4    molar; or-   d) brine salts at a concentration of at least 90% by weight of the    concentration at saturation.

The salts will commonly be inorganic, such as alkali metal, particularlysodium, potassium or caesium, or alkali earth metal particularly calciumor magnesium, or occasionally heavy metals such as zinc, halides,particularly chlorides or bromides, and occasionally nitrates; or watersoluble, particularly alkali metal e.g. sodium or potassium, silicatesor metasilicates. Sodium chloride is very commonly used, and frequently,particularly in marine environments, the drilling fluid may be based onseawater, which includes other salts as well as sodium chloride althoughat relatively low concentrations. Other salts, particularly salts oforganic acids such as formic acid, may be used, for example sodium,potassium and caesium formates. The brine will often include a mixtureof salts as mixtures may enable higher concentration and thus aqueousphase densities to be achieved. Most of these brines are naturallyrelatively near to neutral, though alkali metal earth halides will tendto be moderately acidic and alkali metal, particularly sodium orpotassium, formates tend to be moderately alkaline. Silicate brines areunusual among brines in that the brine is naturally relatively alkaline,generally having a pH>10, more usually about 12. Indeed a major utilityof silicate brines is that then can react to precipitate silica orinsoluble silicate in pores or cracks in the well bore wall, e.g. inshale so they inherently act as shale inhibitors.

Generally the concentration of salt(s) in the brine will depending onthe density desired for the brine. Typically, the concentration will befrom 0.1 to 30%, particularly 0.1 to 10%, by weight of the drillingfluid. For particular salts, concentrations in the brines will moreusually be in the following ranges:

Concentration Broad More usually Brine Salt (% w/w) molar (% w/w) molarNotes sodium chloride 1 to 35 0.2 to 6  2 to 6 0.4 to 6   up to satn ca6.1 M calcium chloride 1 to 20   0.1 to 6.5 0.1 to 20 0.5 to 6.5 up tosatn ca 6.5 M potassium 0.1 to 30   0.1 to 4 0.1 to 10 0.1 to 1.5 up tosatn ca 4.7 M chloride caesium formate  9 to 82*  0.5 to 11  17 to 82  1to 11 up to satn potassium 4 to 45 0.5 to 9   8 to 40 1 to 8 up to satnca 9 M formate sodium silicate 0.5 to 40   0.4 to 2   2 to 20 0.5 to 1  *caesium formate is typically supplied as an 82 wt % solution

Other materials that may be included in the drilling fluid include:

Fluid loss control agents act to reduce the tendency of the drillingfluid to penetrate rocks through which the well is being drilled thuslosing drilling fluid and potentially damaging the rock e.g. bypotentially blocking a production zone and examples include polymerssuch as cellulose polymers, particularly modified e.g. esterified oretherified cellulose. When used, fluid loss control agents are typicallyincluded at levels of 0.1 to 10 wt % based on the drilling fluid.

Shale inhibitors are typically alcohol alkoxylates such as C₃ to C₆alcohol ethoxylates, propoxylates or mixed ethoxylate/propoxylates e.g.butanol mixed ethoxylate/propoxylates, or ethylene oxide/propylene oxideblock copolymers, and, when used, are typically included at levels of0.1 to 10 wt % based on the drilling fluid.

Rheology modifiers or viscosifiers are typically polysaccharides ormodified polysaccharides such as Xanthan gum, starch, polyanioniccellulose and, when used, are typically included at levels of 0.1 to 0.5wt % (100 to 500 ppm) based on the drilling fluid.

Gas hydrate inhibitors can be either thermodynamic inhibitors (meltingpoint depressants) such as glycols such as monoethylene glycol ordiethylene glycol, used at levels of from 1 to 20 wt % based on thedrilling fluid; or kinetic inhibitors, particularly crystallisationinhibitors or crystal growth inhibitors, such as polyvinyl pyrolidone,or quaternary ammonium salts, typically used at levels of from 0.1 to10, more usually 0.2 to 2, wt % based on the drilling fluid.

Dispersants may be included to aid dispersion of solid components of orsolids held in the drilling fluid. Solid components of the drillingfluid include weighting solids cuttings. When used, dispersants aretypically included at levels of 0.1 to 5 wt % based on the drillingfluid.

The overall composition of typically drilling fluid which may be used inthe invention is summarized in the table below:

Material amount (% w/w) brine salts 0.1 to 30 dispersed solids (whenpresent) weighting material   1 to 75 bridging material   1 to 30 fluidloss control agents (when 0.1 to 10 present) shale inhibitors (whenpresent) 0.1 to 10 rheology modifiers (when present)  0.1 to 0.5 gashydrate inhibitors (when present) thermodynamic   1 to 20 kinetic 0.1 to10 dispersant 0.1 to 5  lubricant hydrocarbyl polyether phosphate 0.05to 1  ester polyalkylene glycol 0.5 to 10 water to 100%

In use, re-circulated drilling fluids will tend to accumulate morefinely divided part of the drilling cuttings—the coarser materials willgenerally be removed e.g. by screening before re-circulation. In thissense, drilling cuttings may be regarded as a component of drillingfluids and will typically be present at levels of up to 20, more usuallyup to 10, wt % based on the drilling fluid.

The following Examples illustrate the invention. All parts andpercentages are by weight unless otherwise stated.

Materials PE1 Oleyl alcohol 6EO phosphate ester; ca 80%* mono-ester PE2C8/C10 alcohol 4.5 EO phosphate ester; ca 60% mono-ester PE3 Decyl 4 EOphosphate ester; ca 90%* mono-ester PE4 Decyl 4 EO phosphate ester; ca90%* mono-ester Comparative phosphate ester lubricant: CPE1 2-ethylhexylphosphate ester; ca 50%* mono-ester PAG1 PEG 400 - polyethylene glycolnumber average MW 400 Clay HMP (Hymod Prima Clay) clay used to simulatedrilling cuttings *The % mono-ester figures for the phosphate esters arewt % based on the ester content of the material used.

Test Method(s)

-   Drilling Lubricity—was measured using a journal pin and vee-jaw    “Falex” tester. The test was run with the pin and jaws immersed in    the fluid under test at an initial test load of 250 lbf (ca 1.1 kN)    and held at this force for 15 minutes after which the applied force    was increased until pin failure. The load at failure (in lbf) was    noted and the coefficient of friction calculated.-   Compatibility testing of phosphate esters; the following brines were    used:

Br1 3% NaCl Br2 3% NaCl + 0.05% CaCl2 Br3 3% NaCl + 20% CaCl2 Br4Saturated NaCl Br5 3% NaCl + Na silicate

Alkali was added to brines Br1 to Br4 to adjust the pH to about 11 andto Br5 to adjust the pH to about 12.5. These test brines simulatevarious types of drilling fluid: Br1 a brine having an electrolytestrength similar to seawater; Br2 similar but with the addition of Ca ata level somewhat higher that using hard water or dissolution of Ca fromcarbonaceous rocks; Br3 a mixed sodium chloride-calcium chloride(moderately concentrated) brine; Br4 a saturated sodium chloride brine;and Br5 a silicate brine. The lubricant formulation was added to thebrine at 5% by weight of the brine and mixed into the brine. The mixturewas allowed to stand for 30 mins and the compatibility was assessed andreported using a numerical ranking scale as follows:

ranking description 5 clear liquid 4 slightly cloudy liquid 3 cloudyliquid 2 cloudy liquid showing some cheesing of lubricant 1 substantiallubricant precipitation 0 lubricant substantially completely insolublein brine

EXAMPLE 1

An experimental drilling fluid lubricant formulation Lub1 was made up asfollows:

Material wt % PE1 10 PAG1 75 Water 15

The compatibility of Lub1 was assessed the results are set our in Table1a below:

TABLE 1a Phosphate Brine ester PAG Br1 Br2 Br3 Br4 Br5 PE1 PAG1 5 5 5 55

The lubrication performance of Lub1 at various addition levels from 0 to5% by weight was assessed for load to fail and coefficient of friction(CoF) in four drilling fluids (muds) using a Falex tester and theresults are set out in Table 1b below.

The Muds Used Were:

Material wt % Mud A - a high solids freshwater drilling fluid sodiumcarbonate 0.05 potassium chloride 5 polymeric fluid loss control 0.2additive starch 0.8 Xanthan 0.3 barite 30.1 clay 4 water to 100% Mud B -a drill-in drilling fluid Calcium carbonate 2 calcium chloride 14polymeric fluid loss control 1 additive water to 100% Mud C - potassiumformate drilling fluid calcium carbonate (solid) 9.2 barites 8.3 calciumchloride 1.7 sodium chloride 4.2 potassium formate 12.5 polymeric fluidloss control 1.7 additive Xanthan 0.4 water to 100% Mud D - High calciumdrilling fluid calcium carbonate (solid) 25 calcium chloride 25 sodiumchloride 120 polymeric fluid loss control 10 additive clay 15 water to100%

TABLE 1b Falex results wt % Load to fail Ex No Mud Lub1 (lbf) (N) CoF1.1a Mud A 0 840 3.7 0.22 1.1b 1 2600 11.6 0.18 1.1c 3 >3000 >13.3 0.161.1d 5 >3000 >13.3 0.12 1.2a Mud B 0 — — 0.15 1.2b 1 — — 0.15 1.2c 3 — —0.13 1.2d 5 — — 0.12 1.3a Mud C 0 740 3.3 0.20 1.3b 1 1200 5.3 0.18 1.3c3 1250 5.6 0.16 1.3d 5 >3000 >13.3 0.15 1.4a Mud D 0 900 4 0.19 1.4b 12800 12.5 0.18 1.4c 3 >3000 >13.3 0.15 1.4d 5 >3000 >13.3 0.15

EXAMPLE 2

Different phosphate esters and PAGs were tried by substitution in thefollowing formulations.

Composition of formulations Phosphate Formulation ester PAG type type %type % Water % FG1 various 10 PAG1 75 15 FG2 various 5 PAG1 80 15

The results of compatibility testing are set out in Tables 2.i to 2.iibelow:

TABLE 2.i Formulations FG1 Form Phos. Brine Compatibility No ester PAGBr1 Br2 Br3 Br4 Br5 DrFl 1 PE1 PAG1 5 5 5 5 5 DrFl 2 PE2 PAG1 5 3 3 5 5DrFl 3 PE3 PAG1 5 3 3 5 5 DrFl CPE1 PAG1 3 3 2 3 3 C1

TABLE 2.ii Formulations FG2 Form Phos. Brine Compatibility No ester PAGBr1 Br2 Br3 Br4 Br5 DrFl 7 PE1 PAG1 5 5 5 5 5 DrFl 8 PE2 PAG1 5 3 3 5 5DrFl 9 PE3 PAG1 5 3 3 5 5 DrFl 10 PE4 PAG1 5 3 3 5 5 DrFl CPE1 PAG1 3 32 3 3 C3

1-16. (canceled)
 17. A lubricating agent for a drilling fluid whichcomprises an aqueous solution of at least one mono-(hydrocarbylpolyether) phosphate ester at a concentration of from 0.05 to 1 wt % ofthe solution, and at least one polyethylene glycol having an averagemolecular weight of from 100 to 4000 at a concentration of from 0.5 to10 wt % of the solution.
 18. A lubricating agent as claimed in claim 17wherein the hydrocarbyl polyether phosphate is or includes a compound ofthe formula (I):R¹O.(AO)_(n),P(O)(OH)₂  (1) where R¹ is a C₆ to C₃₀ straight or branchedchain alkyl, alkenyl or alkadienyl group; AO is an alkyleneoxy group;and n is from 2 to
 100. 19. A lubricating agent as claimed in claim 17wherein the group R¹ is a C₈ to C₂₂ straight or branched chain alkyl,alkenyl or alkadienyl group.
 20. A lubricating agent as claimed in 17wherein all the alkyleneoxy groups, AO, are ethyleneoxy (—C₂H₄O—)groups.
 21. A lubricating agent as claimed in 17 wherein the alkyleneoxygroups, AO, are a mixture of ethyleneoxy and propyleneoxy (—C₃H₆O—)groups having a molar ratio of ethyleneoxy groups to propyleneoxy groupsof from 3:1 to 20:1.
 22. A lubricating agent as claimed in claim 17wherein n is at least
 3. 23. A lubricating agent as claimed in 17wherein the concentration of the mono-(hydrocarbyl polyether) phosphateester(s) is from 2 to 40 wt %, the concentration of the polyethyleneglycol is from 50 to 90 wt %, and the concentration of water is from 5to 50 wt %, all based on the total weight of the solution. 24-26.(canceled)
 27. A method of drilling-in a well bore in a productionformation in which a drill-in fluid is circulated through a drill stringextending along the well bore, around a drill bit and returns in thespace between the drill string and the wall of the well bore, in whichthe drill-in fluid is as claimed in claim 16.